The Drying of Spent Coffee Grounds in a Tray Drier

Biodiesel has become an important sustainable alternative to petroleum-based diesel. Spent coffee grounds as a starting material to produce biodiesel can be cost effective especially if the grounds are dried before extracting the coffee oil. This work is part of a comprehensive research project to develop a sustainable biodiesel production process and focuses on the drying of spent coffee grounds using a laboratory tray drier. The drying kinetics were studied using two and four samples with controlled geometries at three different air flow rates and temperatures of 28, 30 and 32°C. The experimental data were used to determine the critical transport parameters in the drying process. Effective diffusivities varied from 1.03x10-11 to 8.11x10-10m2/s. The convective mass transfer coefficient was found to vary from 1.25x10-7 to 3.78x10-6m/s, while the heat transfer coefficient varied from eight to 31.2W/m2°C.


Introduction
The strain on the global economy brought by fossil fuels is astounding. With global energy consumption more than doubling in the past forty years, the proportion of consumption via "finite fuels"; oil, coal, and natural gas, has decreased from 75.9% to a still sizeable 66.5% of all fuel usage [1]. As such, there is a wide area of demand for sustainable fuel sources. It is estimated that the 16.34 billion pounds of coffee produced each year can yield about 340 million gallons of biodiesel via extraction of necessary compounds from Spent Coffee Grounds (SCG), not to mention the average of 3.4% annual growth of coffee production over the last four years [2,3]. Additionally, with the European Union and United States jointly contributing over half of the total world demand of coffee, utilization of this resource could be relatively easy, given the available manufacturing and processing prowess [3].
In the overall process of biodiesel production from Spent Coffee Grounds, it is possible to obtain an oil yield of 10-20% from the SCG and then a near 100% conversion of that oil to biodiesel, with the finished product being stable for over one month when stored under proper conditions [2]. However, the moisture content of the SCG largely dictates the efficiency of this process, from transport concerns to the actual kinetics of the extraction. For example, the only study published so far on the drying of SCG, utilized SCG with starting and equilibrium moisture contents of about 1.41g moisture/g dry mass and 0.0753g moisture/g dry mass, respectively [4]. Thus, properly drying SCG can more-than-double the rate of dry basis transport to treatment facilities for various uses including biodiesel production.
On the chemical side, dry SCG have about double the lipid yield of wet SCG (moisture content of 0.66ml/g) [5]. Therefore, drying the SCG also has impact in the efficiency of the process. The drying of solids has been studied extensively, however, there is only one published study on the drying of SCG [4]. The authors used SCG supplied by a local cafeteria in Jaen Spain but there is no mention about the particle size nor the type of coffee. The study was conducted at relatively high temperatures (100 to 250°C) in a convective drier and at a single drying air velocity. Under these conditions, the results did not show a constant rate drying period and therefore, only effective moisture diffusivities were determined, no heat nor mass transfer coefficients were obtained.
Extensive fruit and agricultural products drying studies have been published in which both moisture diffusivities and heat and mass transfer coefficients were determined. Pioneer work was pub-lished by Dost in 1995 and1996; in the first study [6], the authors developed an analytical model to determine the moisture diffusivities in geometrical solid objects (infinite slab, infinite cylinder and sphere). New correlations for the roots of the characteristic equations corresponding to these objects were developed; for a drying application, using the similarity between cooling and drying profiles, the drying coefficient and the lag factor are obtained. The model was verified using available data and the results showed that the model can be used to determine moisture diffusion coefficients in such solid objects in a simple and accurate manner for a variety of drying applications.
In the second publication [7], Dincer and Dost presented an effective analytical model to determine the moisture diffusivities and moisture transfer coefficients for solid objects, shapes mentioned above, as well as for irregularly shaped objects by using a shape factor for drying applications. The analytical model was verified using available experimental data and the results showed that the method can be used to determine the moisture diffusion coefficients and moisture transfer coefficients for such solid objects for a variety of drying applications. Later, in a relevant contribution, Dincer et al. [8] developed a new Biot number-drying coefficient correlation that is useful to determine the moisture transfer parameters in terms of moisture diffusivity and moisture transfer coefficient involved in the solids drying process.
In a later work, Dincer and Hussain [9] developed an even more useful and generalized Biot number-lag factor correlation applicable to predict moisture transfer parameters for three regular shaped solids, slab, cylinder and sphere. More recently, Mullen et al. [10] published results for the drying of apple slices in a tray drier at several temperatures and drying air velocities; moisture diffusivity and heat and mass transfer coefficients were reported that are in very good agreement with previous studies. Velic et al. [11] investigated the effect of airflow velocity in the range from 0.64 to 2.75m/s on the kinetics of convection drying of Jonagold apple, and on the heat transfer coefficient and on the effective diffusivity. The experiments were conducted in a convection tray drier at 60oC temperature using rectangle-shaped (20x20x5mm) apple samples. found that the effective diffusivity increased with temperature from 4.4x10-10 at 30°C to 1.4x10-9 m 2 /s at 60°C, while the mass transfer coefficient increased from 307x10-10 to 7.4x10-9m/s in the same temperature range [13,14].
The purpose of the following experimental pursuit is to understand the drying properties of SCG in order to extrapolate fundamental properties about said process for application in the practical production of biodiesel from SCG. Additionally, the findings from this study can be applied to facilitate the transport of coffee grounds for any specific use. The temperature and humidity values were measured using a hand-held thermo-hygrometer. An anemometer was used to measure flow velocity and it was important for to insert the flowmeter with the same depth each run, in order to obtain reliable data.

Materials
While the temperature was set to 100°F, the temperature at the point of sample content was 10-20°F lower; this is probably due to heat loss through the sides of the duct. However, it is assumed that this heat loss does not affect the temperature profile of the air that interacts with the sample. As for the air flow, mean inlet air flow seemed weakly-dependent if not independent of the position of the analog, mark-less dial used to set it after a certain point ("noon").
The dry basis for each coffee grounds cake was found by putting them in an Isotemp 100L Convective Oven (Fisher Scientific) at 70°C for 24 hours and measuring the weight of the dry sample cakes. An average moisture content of 68.2% (wet basis) was determined. The same procedure was used to determine the equilib-rium moisture content that was 7% (wet basis) for an ambient air with a 37% relative humidity at 22°C. The trays relate to the digital scale that displays the weight of coffee grounds. The precision of the mass balance is +/-0.5 grams

Assumptions
The airflow is assumed to be well mixed; bulk flow is unidirectional and parallel to the walls of the duct. The only process occurring is the evaporation of water into a passing airstream. Despite the relatively poor precision of the balance inside of the duct dryer (+/-0.5g), it is assumed that the precision is enough to allow identification of possible constant rate drying periods. During a given constant rate drying period, it can be assumed that the interface temperature is equal to the downstream wet bulb temperature [8].
When calculating the convective heat transfer coefficient, assume that the upstream flow properties are more descriptive of the actual contact system than those downstream.

Control of Environmental Variations
While an ideal system would reach a stable steady state throughout the duration of the experiment, the apparatus has an on/off control of the heater. In short, falling a few degrees below the temperature of interest would activate the air heater, and exceeding a similar amount of degrees above the temperature of interest shuts down the heater. This results in a sinusoidal fluctuation in temperature-dependent variables, including the dry bulb temperature, wet bulb temperature, humidity, and, to an extent, the flow rate. Therefore, a uniform approach to collecting measurements was needed while still accounting for the overall trend. In order to account for the fluctuation, a two-minute video recording of each oscillating measurement was taken after the measuring device had been within the steady-state dryer long enough to not be affected by anything except for the dryer.  it is possible to, not only predict where most of the error is coming from (air speed and humidity) but also why that category has a significant amount of error (irregular airflow, which may also affect the humidity in terms of flow patterns).
The shifts within all the sine functions are specific to the calibration data. The important values worth noting are: A.
The pre-sinusoidal coefficient (amplitude), which states how far typical measurements could range from the mean.
B. The period, which gives an idea of how frequently the system oscillates (note how the denominators of the period are all approximately the same within about 4%).

C.
The y-intercept, which is the mean of the function, not of the experimental data (although the two are almost the same).
However, it is possible to approximate the mean for a given periodic variable by measuring the variable's value at its maximum at any given interval and then subtracting the corresponding amplitude above from that value. This method was used for all four categories above.

Equations and Results
In the coffee drying process, heat and mass transfer occur si- The dimensionless moisture ratio is calculated using the following equation: Where: X R = dimensionless moisture ratio at time t X t = moisture content at time  Figure 4 shows these CRDP's for both experimental runs.
The following method and equations were used to find the experimental mass and heat transfer coefficients.

Copy@ Enrico N Martinez
American Journal of Biomedical Science & Research 420   The mass transfer calculation via the use of correlations begins with plotting dimensionless moisture content ratio versus drying time. The plot is shown in Figure 5 below. The data is fitted to an exponential function as follows [7]: ( ) Where: The lag factor is used in the following equation to determine the To obtain the moisture diffusivity in the coffee grounds cakes, we can describe the process by means of Fick's second law of diffusion applied to the one-dimensional mass transfer in an infinite slab geometry [4], and taking only the first term of the series solution to the differential equation as follows: Taking the natural logarithm on both sides in equation (9): The equation above shows that the diffusivity, D e , can be obtained from the slope of the graph in (Figure 6) below.  A table with the summary of experimental results obtained for each run in the first series of experiments is shown in Table 2. It is worth noting that the "experimental" heat transfer coefficient for these results was calculated using the following correlation for the drying of solids in a tray drier [15]: Evidently, there is a considerable difference in the effective diffusivity found in this study in comparison with the result published by Gomez-de la Cruz et al. [4]. The sample thickness was the same in both cases but clearly, the air temperature and flow rate were higher in the published study. Heat and mass transfer coefficients are comparable to those published for apple drying studies in the same laboratory tray drier [8]. It is evident that the experimental conditions were very limited, only one air temperature was tested, and the range of airflow rate had very little variation.
Therefore, a new series of experiments was planned, and performed, by a team of three researchers, in contrast with the original experiments that were run by a single researcher. In this case, four wet SCG cakes 5.08cm in diameter by 2.54cm thick and weighing 25 grams each were placed in the four trays inside the drier; the surface area available for drying was effectively increased in this series of experiments. Four experimental runs were carried out and the results are shown in Figure 7, the experimental conditions are given in the Table 3.     There are no published results for heat and mass transfer coefficients in the case of drying of SCG. Some comparison can be made against values obtained in the drying of fruits [8,11]. Such values are included in

Conclusions and Future Directions
Despite the setbacks, there are still a notable number of pos-describes, in detail, the calculations used and the order in which they were used, allowing future interests in this area to be more quickly carried out. Secondly, this analysis also includes a thorough approach in accounting for the periodic variance in dual-threshold systems that also assists in isolating possible sources of error or irregularity (in this case, the flow speed). Performing this analysis allows for a data set that is better suited to the properties of the environment averaged across time.
The effective diffusivity obtained using the same method as the most recent literature source produced a somewhat different result than the published value. However, this makes sense given the difference in environment between our experiments and those previously published [4]. Moreover, it is worth noting that the temperatures used in the study by De la Cruz et al. [4] were so high that there is the possibility for evaporation of other components besides water from the SCG samples, something that might contribute to the observed differences in diffusivities.
The convective heat and mass transfer coefficients are the first to be determined experimentally for the drying of SCG and the values obtained are in a reasonable agreement with those reported for the drying of fruits and foodstuff. These transfer coefficients are useful to design industrial size driers, such as continuous tunnel driers or tray driers, depending on the volume of SCG to dry.
In building upon the above findings, there are several pertinent areas to consider. First, particle size is of large concern because the grounds swell depending on their water intake.
In other words, as a particle loses water, it decreases in size, not to mention it is less prone to clumping together with other particles. While particle size was briefly studied using sieve trays of various sizes, time was quickly allocated to other experiments when it became apparent that pinpointing particle size to a specific moisture content would prove difficult. In other words, a sample would lose up to half of its starting weight throughout the duration of a single particle size test because the water would quickly evaporate at ambient temperatures due to the largely increased surface area of drying.
Additional testing could be more mindful of the limitations of the equipment. By selecting lower analog settings for the air flowrate, it would then be possible to perform tests at airflows that consistently differ from other trials if the lower settings all work within the capacity of the apparatus. Improvements concerning particle size study and proper airflow bounding were not implemented in the above analysis due to time constraints, but they are certainly feasible in the future.